This invention relates to an exposure method, an exposure apparatus and a device manufacturing method and, more particularly, to a multiple exposure technology for lithographically printing a very fine circuit pattern on a photosensitive substrate. The present invention is suitably usable, for example, in the production of various patterns to be used for various devices and micro-mechanics such as semiconductor chips (e.g., ICs or LSIs), display devices (e.g., liquid crystal panels), detecting devices (e.g., magnetic heads), or image pickup devices (e.g., CCDs), for example.
Generally, the manufacture of microdevices such as ICs, LSIs or liquid crystal panels through photolithographic processes uses a projection exposure method and a projection exposure apparatus wherein a circuit pattern formed on a photomask or a reticle (hereinafter, xe2x80x9cmaskxe2x80x9d), for example, is projected to and thus transferred (photoprinted) to a photosensitive substrate such as a silicon wafer or a glass plate (hereinafter, xe2x80x9cwaferxe2x80x9d), for example, through a projection optical system.
With increasing density (integration) of these devices, further miniaturization of a pattern to be transferred to a wafer is required. Namely, improvements in the resolution as well as enlargement of the area of a single chip on the wafer are required. In this respect, projection exposure apparatuses and projection exposure methods which are the core of the wafer micro-processing technology are required to assure formation of an image of a size (linewidth) of 0.1 micron or less in a wider region, particularly, a circuit pattern of 80 nm or less.
FIG. 33 is a schematic view of a general structure of a projection exposure apparatus. Denoted in the drawing at 191 is an excimer laser which is an exposure light source. Denoted at 192 is an illumination optical system, and denoted at 193 is illumination light. Denoted at 194 is a mask, and denoted at 195 is object-side exposure light emitted from the mask 194 and impinging on a reduction projection optical system 196. Denoted at 197 is image-side exposure light emitted from the optical system 196 and impinging on a photosensitive substrate 198 which is a wafer. Denoted at 199 is a substrate stage for holding the photosensitive substrate.
The laser light emitted from the excimer laser 191 is directed by a guiding optical system to the illumination optical system 192, by which the light is adjusted and transformed into the illumination light 193 having a predetermined light intensity distribution, orientation distribution, opening angle (numerical aperture NA) and the like. The illumination light 193 illuminates the mask 194. The mask 194 has a fine pattern corresponding to a fine pattern to be produced on the wafer 198. The mask pattern is formed on a quartz substrate by use of chromium, for example, and the pattern has a size corresponding to the inverse of the projection magnification of the projection optical system, that is, 2xc3x97, 4xc3x97 or 5xc3x97, for example. The illumination light 193 is transmissively diffracted by the fine pattern of the mask 194, whereby object-side exposure light 195 is provided. The projection optical system 196 functions to transform the object-side exposure light 195 into image-side exposure light 197 for imaging the fine pattern of the mask 194 upon the wafer 198 in accordance with the above-described projection magnification and with a sufficiently small aberration. As best seen in an enlarged view at the bottom of FIG. 33, the image-side exposure light 197 is converged upon the wafer 198 with a predetermined numerical aperture NA (=sinxcex8), whereby an image of the fine pattern is produced on the wafer 198. When fine patterns are to be produced sequentially on different regions of the wafer, each corresponding to single or plural chips, the substrate stage 199 moves stepwise along the image plane of the projection optical system, to change the position of the wafer 198 with respect to the projection optical system 196. Denoted at 200 is a pupil position of the projection optical system.
With currently prevailing projection exposure apparatuses having an excimer laser as a light source, such as described above, however, it is not easy to produce a pattern of a linewidth of 0.15 micron or narrower.
In the projection optical system 196, there is a limit of resolution due to the tradeoff between the depth of focus and the optical resolution attributable to the exposure wavelength (wavelength used for the exposure process). The resolution R of a resolving pattern and the depth of focus DOF in a projection exposure apparatus can be expressed by Rayleigh""s equations such as equations (1) and (2) below.
R=k1(xcex/NA)xe2x80x83xe2x80x83(1) 
DOF=k2(xcex/NA2)xe2x80x83xe2x80x83(2) 
where xcex is the exposure wavelength, NA is the image-side numerical aperture of the projection optical system which represents the brightness thereof, and k1 and k2 are constants which are determined by the developing process characteristic of the wafer 198, for example, and which are generally about 0.5 to 0.7. From equation (1) above, it is seen that improvements in resolution (making the value of resolution R smaller) can be attained by enlarging the numerical aperture NA (NA enlargement). Also, from equation (2), it is seen that improvements in resolution anyway necessitate shortening the exposure wavelength (wavelength shortening) because the projection optical system 196 should have a certain value of the depth of focus DOF in the practical exposure such that the NA enlargement cannot be done unlimitedly.
However, the wavelength shortening encounters a critical problem. Namely, there is no glass material usable for the projection optical system 196. Most glass materials have a substantially zero transmission factor with respect to the deep ultraviolet region. Fused silica is a glass material which can be produced by a special process, for use in an exposure apparatus having an exposure wavelength of about 248 nm. However, even the transmission factor of fused silica largely decreases with respect to the exposure wavelength of 193 nm or shorter. In the exposure wavelength region of 150 nm or shorter, corresponding to a fine pattern of 0.15 micron or less, development of a practical glass material is very difficult. Further, glass materials to be used in the deep ultraviolet region should satisfy not only the transmission factor but also various conditions such as durability, refractive index uniformness, optical distortion, and machining easiness, for example. In consideration of these factors, development of practical glass materials is difficult to accomplish.
As described above, in conventional projection exposure method and projection exposure apparatuses, the wavelength shortening to an exposure wavelength of about 150 nm or less is necessary to enable formation of a pattern of 0.15 micron or narrower on a wafer, whereas there is no practical glass material to be satisfactorily used in such a wavelength region.
A dual exposure method has been proposed by the same assignee of the subject application, which method comprises a combination of a dual-beam interference exposure and a multiple-value exposure, to enable formation of a pattern of 0.15 micron or less with use of a currently available exposure apparatus.
The dual exposure method is a method in which a multiple value exposure process based on an ordinary or standard exposure as well as a periodic pattern exposure process based on a dual beam interference exposure are performed without intervention of a development process. More specifically, a periodic pattern may be photoprinted with a level less than the exposure threshold level of a resist and, thereafter, a standard exposure process having a multiple level exposure amount distribution is performed. As regards the exposure amount by the standard exposure, different exposure amount distributions are produced in different small zones of the exposure pattern region (exposure region). Here, the exposure amount refers to an exposure amount on a resist. The exposure amount distribution may include a multiple-value exposure amount distribution, that is, a region with an exposure amount not less than the resist exposure threshold value and a region with an exposure amount not greater than the same resist exposure threshold value.
Referring to an example of a gate pattern shown in FIG. 3, a circuit pattern (lithography pattern) to be produced by the exposure will be explained. The gate pattern of FIG. 3 has a smallest lateral linewidth of 0.1 micron, which is beyond the range of resolution by the standard exposure process. On the other hand, it has a smallest longitudinal linewidth of 0.2 micron or more, which is within the range of resolution by the standard exposure process. In accordance with the dual exposure method, as regards a two-dimensional pattern, as above, having a smallest linewidth pattern for which a high resolution is required only with respect to a one-dimensional direction (i.e., lateral direction), the periodic pattern exposure based on the dual-beam interference exposure, for example, may be performed with use of only a periodic pattern in a one-dimensional direction in which a high resolution is required.
FIGS. 1A-1C show exposure amount distributions in each step of the dual exposure process. Numerals in these drawings each denotes an exposure amount of the resist. FIG. 1A shows an exposure amount of the resist. FIG. 1A shows an exposure amount distribution provided by a periodic exposure pattern, wherein repetition patterns are formed in a one-dimensional direction. The exposure amount at blank (white) portions other than the pattern is zero. The pattern portion has an exposure amount of 1.
FIG. 1B shows an exposure amount distribution provided by a multiple-value standard exposure. The exposure amount at blank portions other than the pattern is zero. The pattern portion has a multiple-value exposure amount distribution (dual-value in this example) of 1 and 2.
When these exposures are performed (dual exposure) without intervention of a development process, the result is that a distribution corresponding to the sum of these exposure amounts is produced on the resist, whereby an exposure amount distribution such as shown in FIG. 1C is provided. Here, when the exposure threshold value of the resist is between 1 and 2, the portion with an exposure amount larger than 2 is sensitized (printed) such that a pattern depicted by a thick line in FIG. 1C can be produced, with a development process. The exposure pattern which is outside the thick-line portion and which is provided by the periodic pattern exposure has an exposure amount less than the resist exposure threshold value and, therefore, it disappears through the development process.
As regards the portion where exposure amounts by the standard exposure and being not greater than the resist exposure threshold value are distributed, those regions where the sum of the exposure patterns provide by the standard exposure and the periodic pattern exposure is not less than the resist exposure threshold value are developed into a pattern by the development process. Thus, at the portion where the exposure patterns provided by the standard exposure and the periodic pattern exposure are superposed one upon another, an exposure pattern having the same resolution as that of the exposure pattern of the periodic pattern exposure is produced.
As regards the exposure pattern region where exposure amounts provided by the standard exposure and being not less than the resist exposure threshold are distributed, although exposure patterns of the standard exposure and the periodic pattern exposure are superposed, an exposure pattern having the same resolution as the exposure pattern of the standard exposure is produced.
FIGS. 2A-2C show patterns and masks for producing exposure amount distributions of FIGS. 1A-1C. FIG. 2A shows a pattern and a mask for producing a repetition pattern only in a one-dimensional direction in which a high resolution is required. For example, a Levenson type phase shift mask may be used. If a Levenson type mask is used, blank (white) portions and gray portions in the drawing have mutually inverse phases. Due to the effect of the phase inversion, a periodic exposure pattern of high contrast can be produced through a dual-beam interference exposure process. The mask is not limited to the Levenson type phase shift mask. Any type of mask may be used provided that an exposure amount distribution such as above can be produced thereby.
This exposure pattern has a period of 0.2 micron, for example, and it comprises a line-and-space pattern having a linewidth of 0.1 micron (each line and each space). The exposure amount distribution shown in FIG. 1A is produced by this.
As regards a pattern and a mask for forming a multiple-value pattern, a mask having formed thereon a pattern being analogous to a circuit pattern to be produced finally is used. In this case, a mask having a gate pattern formed thereon, as shown in FIG. 2B, is used.
As described hereinbefore, the portion of a gate pattern which comprises a very fine line of 0.1 micron width is a pattern narrower than the resolution of the standard exposure. As a result, two narrow-line portions are not resolved, but a uniform low-intensity distribution is produced. As compared therewith, patterns at the top and bottom ends of the gate pattern can be resolved as high-intensity patterns because the linewidth thereof is inside the resolution range of the standard exposure.
Thus, when the pattern and the mask of FIG. 2B are printed, the result is that a multiple-value exposure amount distribution such as shown in FIG. 1B is produced.
In this example, the pattern to be produced is of a light transmission type. A light blocking type pattern can be produced by use of a mask such as shown in FIG. 2C. Namely, a light blocking type pattern can be formed by using a mask arranged so that light is transmitted through a region other than the pattern, whereas light is blocked by the pattern.
In the case of a light blocking type pattern, a pattern beyond the resolution blocks the light and the exposure amount distribution becomes zero, whereas a fine pattern less than the resolution does not completely block the light, and an exposure amount about a half of the exposure amount distribution around the pattern is provided. As a result, a multiple-value exposure amount distribution is produced.
The principle of the dual exposure process can be summarized as follows.
1. The periodic pattern exposure region not fused with the exposure pattern of standard exposure, that is, the periodic pattern not greater than the resist exposure threshold value, disappears with the development process.
2. As regards the pattern region of standard exposure having been exposed with an exposure amount not greater than the resist exposure threshold value, an exposure pattern corresponding to a portion of a desired circuit pattern to be produced and having the same resolution as that of the periodic pattern exposure, is produced. This exposure pattern is determined on the basis of the combination of exposure patterns to be provided by the standard exposure and the periodic pattern exposure, respectively.
3. As regards the pattern region of standard exposure having been exposed with an exposure amount not less than the resist exposure threshold value, an exposure pattern corresponding to the mask pattern is produced.
The multiple exposure method described above has an additional advantage. That is, if the periodic pattern exposure for which a highest resolution is required is required is performed through the dual-beam interference exposure using a phase shift mask, for example, a large depth of focus is obtainable. Further, as regards the order of the periodic pattern exposure and the standard exposure, the periodic pattern exposure is made first in the above-described examples, the order may be reversed or, alternatively, these exposures may be made at the same time.
In the multiple exposure procedure such as described above and as regards the exposure of a large pattern not less than the resolution, being included in the standard exposure pattern, there are a portion to be superposed with a periodic pattern and a portion not superposed therewith. These portions should be similarly formed into a pattern. Thus, concerning a large pattern not less than the resolution, the standard exposure may be dominant.
However, with such standard exposure, a large depth of focus may not be obtainable for a pattern of a size near the resolution. There are cases wherein a fine pattern can be resolved with a large depth, by use of the periodic pattern exposure.
Namely, depending on the exposure condition, there may be cases wherein, as a result of a combination of the standard exposure and the periodic pattern exposure, since the depth of focus with a large pattern not less than the resolution of the standard exposure is smaller than that of a fine line less than the resolution of the periodic pattern exposure, a large pattern is degraded more, by defocus, than that of the fine pattern.
Degradation of the image performance due to aberration is particularly serious in the point of pattern shift mainly caused by asymmetric aberration. As the pattern becomes close to the limit resolution, it becomes more serious in dependence upon the linewidth. Further, there is lateral (horizontal) linewidth asymmetry which becomes very notable with a linewidth k1=0.5 or less, although this has not yet been considered as a problem. Here, the linewidth R is expressed by k1, as divided by (xcex/NA). Hereinafter, the linewidth will be expressed by k1.
Further, because of a proximity effect becoming larger and because of the difference between the linewidth of a pattern at the central portion of the line-and-space pattern and a pattern being almost isolated, the dispersion of the linewidth becomes large.
When the linewidth k1 is not greater than 0.5, the image performance is degraded more. However, because the lateral linewidth asymmetry becomes large, the lateral linewidth asymmetry can be used as one evaluation reference for the image performance.
Thus, by using a gate pattern of FIG. 12 with a two-line pattern, as an evaluation pattern, the lateral linewidth asymmetry may be evaluated and the image performance may be expressed thereby.
xe2x80x9clateral linewidth asymmetryxe2x80x9d=(xe2x80x9cleft linewidthxe2x80x9dxe2x88x92xe2x80x9cright linewidthxe2x80x9d)/(xe2x80x9cpredetermined linewidthxe2x80x9d)xc3x97100 xe2x80x83xe2x80x83(3) 
Here, the predetermined linewidth means the linewidth of the original pattern. It may be a linewidth where no aberration is present. If there is an aberration, it may be an average of the left and right linewidths.
Now, while taking coma aberration as an example, degradation of the image performance will be described. When a wavefront aberration WA is expressed by using Zemike""s coefficient Ci, we obtain:
WA=xc2x7xcexa3CiUi(r, xcex8)xe2x80x83xe2x80x83(4) 
Here, if the coordinate system of a pupil is expressed by polar coordinates r and xcex8, a lower-order coma aberration component, for example, is:
C8(3r3xe2x88x922r)sinxcex8xe2x80x83xe2x80x83(5) 
As an example when the image performance is degraded by aberration, an exposure apparatus having a KrF excimer laser having a wavelength of xcex=0.248 micron and a projection optical system of NA=0.60 and with a coma aberration of c8=0.05xcex is used to expose a pattern having two fine lines of k1=0.3 as shown in FIG. 12 to form an image thereof on the wafer surface.
When a binary reticle of light transmission type with a single phase and having a pattern including two fine lines of 0.13 micron (k1=0.315) such as shown in FIG. 14 is used to perform the exposure, an image such as shown in FIG. 20 is obtainable. Even if there is aberration, substantially no lateral asymmetry occurs. In fact, the asymmetry according to the definition of equation (3) was 2.75%. The illumination condition in that case was "sgr"=0.8 in terms of the effective light source distribution on the pupil plane as shown in FIG. 13. However, only a contrast of 14% was obtained between the two lines, and they were not resolved.
When a phase shift reticle having two phases of 0 and xcfx80 and having a pattern with two fine lines of 0.13 micron (k1=0.315) such as shown in FIG. 15 is used to perform the exposure, an image such as shown in FIG. 21 is obtainable. A contrast of 90% or higher was obtained between the two lines, but the shape was much degraded by aberration. The asymmetry according to equation (3) was as large as 30%. The illumination condition in that case was a "sgr"=0.3.
In FIGS. 14 and 15, the black-painted portion depicts a portion where the light is blocked, while a blank (white) portion depicts a portion where the light is transmitted. Also, in the pattern shown in FIG. 15, two portions for transmitting light have phases being mutually inverted.
It is seen from the above that, in a process wherein the k1 factor is small, aberration should be suppressed to prevent adverse influence to the image performance.
It is an object of the present invention to provide an exposure method, an exposure apparatus and/or a device manufacturing method by which satisfactory resolution is attainable with a multiple exposure process, by which, particularly, the depth of focus can be enlarged in accordance with a pattern to be resolved and with a required depth, and by which the influence of aberration to the image performance can be reduced even in a process where the k1 factor is small.
In accordance with an aspect of the present invention, there is provided an exposure method in which a multiple exposure process including a first exposure for a first pattern and a second exposure for a second pattern is performed by use of a projection optical system to thereby resolve a desired pattern, characterized in that: a numerical aperture NA1 of the projection optical system for the first pattern exposure and a numerical aperture NA2 of the projection optical system for the second pattern exposure are made different from each other.
In one preferred form of this aspect of the present invention, for a common numerical aperture, the first pattern exposure may be performed with a substantially higher resolution and a substantially larger depth of focus, as compared with the second pattern exposure, and the numerical aperture NA2 for the second pattern may be made smaller than the numerical aperture NA1 for the first pattern.
The numerical aperture NA2 for the second pattern may satisfy a relation 0.7 NA1xe2x89xa6NA2 less than NA1.
The numerical aperture NA1 for the first pattern may be determined in accordance with the following equation:
NA1=k1(xcex/R) where k1xe2x89xa70.25 
wherein R is the length of a shorter one of a line and a space of a thinnest portion of the desired pattern, and xcex is the wavelength of light used for the first pattern exposure.
The numerical aperture NA1 for the first pattern may correspond to a largest numerical aperture of the projection optical system.
The first pattern exposure and the second pattern exposure may be performed with different coherence factors "sgr".
In the multiple exposure process, in accordance with a depth of focus required for resolution of the desired pattern, a combination of a coherence factor "sgr"1 and the numerical aperture NA1 of the projection optical system for the first pattern exposure and a combination of a coherence factor "sgr"2 and the numerical aperture NA2 of the projection optical system for the second pattern exposure may be set, respectively.
The coherence factor "sgr"1 for the first pattern exposure may be determined in accordance with a depth of focus required for resolution of a fine line of the width R.
For a fine line having a normalized linewidth k1, obtainable by dividing the width R of the fine line by (xcex/NA1), being not greater than 0.4, the coherence factor "sgr"1 may be made not greater than 0.3.
The numerical aperture NA2 for the second pattern may be determined in accordance with a linewidth of a pattern included in the second pattern, having a linewidth not smaller than the pitch of a periodic pattern of the first pattern and not being dependent upon the first pattern in the resolution.
The first pattern may be constituted by a periodic pattern only a thin portion of which is resolvable, while the second pattern may be constituted by a pattern having a thin portion which is not resolvable, such that, in combination of them, an influence of aberration of the projection optical system is reduced.
The first pattern exposure may use coherent illumination, while the second pattern exposure may use incoherent illumination.
The periodic pattern may include a thin portion corresponding to a thin portion of the desired pattern and a periodic pattern having periods, more than one period, defined at the opposite sides of the thin portion.
In the first pattern exposure, the following relations may be satisfied:
k1xe2x89xa60.4
k2xe2x89xa70.5
where k1 is a value obtainable by dividing, by xcex/NA1, the length R of a shorter one of a line and a space of the thin portion, and k2 is a value obtainable by dividing a focus amount d by xcex/NA12, wherein xcex is the wavelength of light used for the exposure.
When a phase shift mask is used for the first pattern exposure, the coherence factor "sgr"1 may be made not greater than 0.3.
The second pattern exposure may be based on a triple-beam interference imaging process.
The numerical aperture NA2 of the projection optical system for the second pattern exposure may satisfy a relation NA2xe2x89xa60.4xc3x97(xcex/R) where xcex is the wavelength of light used for the exposure, and R is the linewidth.
The numerical aperture NA2 of the projection optical system for the second pattern exposure may satisfy a relation NA2xe2x89xa70.5xc3x97(xcex/R2) where R2 is the linewidth of a portion of the desired pattern, excluding the thick portion not to be resolved by the second pattern exposure but to be resolved by the first pattern exposure, and xcex is the wavelength of light used for the exposure.
The first pattern may be provided by one of a phase shift mask and a mask manufactured by a dual-beam interference process.
The second pattern exposure may use a mask having a pattern of a shape similar to the desired pattern.
The coherence factor "sgr"2 for the second pattern may be set to a largest value obtainable with an exposure apparatus.
Just before the first pattern exposure and the second pattern exposure, an optimum numerical aperture NA may be set by changing an aperture diameter of a circular aperture at a pupil position of the projection optical system.
Just before the first pattern exposure and the second pattern exposure, one of an optical system of the projection optical system and a stop at a pupil position thereof may be moved to set an optimum coherence factor "sgr"1 or "sgr"2.
In accordance with another aspect of the present invention, there is provided an exposure apparatus having an exposure mode for performing an exposure method as recited above.
In accordance with a further aspect of the present invention, there is provided a device manufacturing method, comprising the steps of: exposing a wafer to a pattern of a reticle by use of an exposure method as recited above, or by use of an exposure apparatus as recited above; and developing the exposed wafer.
In accordance with a yet further aspect of the present invention, there is provided an exposure method for performing an exposure of a resist in relation to a desired pattern by use of a projection optical system, said method comprising: a first step for applying a first exposure amount distribution on the basis of a dual-beam interference exposure process; and a second step for applying a second exposure amount distribution on the basis of an exposure process using a mask having a pattern being analogous to the desired pattern, said second exposure amount distribution including a first portion of a small exposure amount not being zero and a second portion of a large exposure amount; wherein an exposure process for a portion of the desired pattern is performed on the basis of superposition of a portion of the first exposure amount distribution and the first portion of the second exposure amount distribution, while an exposure process for the remaining portion of the desired pattern is performed on the basis of the second portion of the second exposure amount distribution as superposed with another portion of the first exposure amount distribution; wherein the numerical aperture of the projection optical system in the second step is made smaller than that of the projection optical system in the first step.
In accordance with a still further aspect of the present invention, there is provided an exposure method for performing an exposure of a resist in relation to a desired pattern by use of a projection optical system, said method comprising; a first step for applying a first exposure amount distribution having an exposure amount not greater than an exposure threshold value of the resist, on the basis of a dual-beam interference exposure process using a first mask having at least one of a phase shifter and a light blocking portion; and a second step for applying a second exposure amount distribution on the basis of an exposure process using a second mask having a pattern being analogous to the desired pattern, said second exposure amount distribution including a first portion of an exposure amount not being zero but being not greater than the exposure threshold value, and a second portion of an exposure amount not less than the exposure threshold value; wherein an exposure process for a portion of the desired pattern is performed on the basis of superposition of a portion of the first exposure amount distribution and the first portion of the second exposure amount distribution, while an exposure process for the remaining portion of the desired pattern is performed on the basis of the second portion of the second exposure amount distribution as superposed with another portion of the first exposure amount distribution; wherein the numerical aperture of the projection optical system in the second step is made smaller than that of projection optical system in the first step.
In accordance with a yet further aspect of the present invention, there is provided an exposure method for performing an exposure of a resist in relation to a desired pattern by use of a projection optical system, said method comprising; a first step for applying a first exposure amount distribution on the basis of a periodic pattern exposure process; and a second step for applying a second exposure amount distribution on the basis of an exposure process using a mask having a pattern being analogous to the desired pattern, said second exposure amount distribution including a first portion of a small exposure amount not being zero and a second portion of a large exposure amount; wherein an exposure process for a portion of the desired pattern is performed on the basis of superposition of a portion of the first exposure amount distribution and the first portion of the second exposure amount distribution, while an exposure process for the remaining portion of the desired pattern is performed on the basis of the second portion of the second exposure amount distribution as superposed with another portion of the first exposure amount distribution; wherein the numerical aperture of the projection optical system in the second step is made smaller than that of the projection optical system in the first step.
In accordance with a still further aspect of the present invention, there is provided an exposure method for performing an exposure of a resist in relation to a desired pattern by use of a projection optical system, said method comprising: a first step for applying a first exposure amount distribution having an exposure amount not greater than an exposure threshold value of the resist, on the basis of a periodic pattern exposure process using a first mask having at least one of a phase shifter and a light blocking portion; and a second step for applying a second exposure amount distribution on the basis of an exposure process using a second mask having a pattern being analogous to the desired pattern, said second exposure amount distribution including a first portion of an exposure amount not being zero but being not greater than the exposure threshold value, and a second portion of an exposure amount not less than the exposure threshold value; wherein an exposure process for a portion of the desired pattern is performed on the basis of superposition of a portion of the first exposure amount distribution and the first portion of the second exposure amount distribution, while an exposure process for the remaining portion of the desired pattern is performed on the basis of the second portion of the second exposure amount distribution as superposed with another portion of the first exposure amount distribution; wherein the numerical aperture of the projection optical system in the second step is made smaller than that of the projection optical system in the first step.
In these aspects of the present invention, the exposure process may be performed in accordance with one of a first procedure in which the second step is carried out after the first step, a second procedure in which the first step is carried out after the second step, and a third procedure in which the first and second steps are carried out simultaneously.
In accordance with a further aspect of the present invention, there is provided an exposure apparatus having an exposure mode for executing an exposure method as recited above.
In accordance with a yet further aspect of the present invention, there is provided a device manufacturing method, comprising the steps of; exposing a wafer to a device pattern by use of an exposure apparatus as recited above; and developing the exposed wafer.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.